drm/vmwgfx: Fix host message module function declarations

Make the host message module function declarations similar to the other
declarations in vmwgfx_drv.h and include the header in vmwgfx_msg.c

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>